Need alternative way to specify no memory request

[gdevenyi]

Currently, memory request of 0 is used to indicate not making a memory request.

SLURM reserves 0 to a special request of "give me all ram on the node". So we can't see this.

Either -1 or maybe start with it being "None"?

[bcdarwin]

'None' is the Python-ism for this, the annoyance being that this doesn't parse as a number so the user can't specify it explicitly (e.g., to override an environment var via the command line) without a couple extra lines of code.